The effect of Pb-substitution on the crystal structures and current-voltage characteristics in the c-direction of Bi 2 Sr 2 CaCu 2 O y . (BSCCO) and (Bi 1 - x Pb x 2 Sr 2 CaCu 2 O y (BPSCCO) (with x ≤ ∼ 0.25) single crystals is investigated. The Bi-type incommensurate modulation structure of the BSCCO single crystal is confirmed, and the modification and then disappearance of this structure with an increase in Pb content in the BPSCCO single crystals are observed. For mesas of these single crystals which exhibit multiple branching in their I-V characteristics, and hence are composed of stacked intrinsic Josephson tunnel junctions, the critical current densities J c are discussed with respect to the structural modulation, suppressed energy gaps Δ and normal resistances R n on the basis of the Ambegaokar-Baratoff relation.